Staphylococcus aureus is a well-known human pathogen associated with surgical site infections, bloodstream infections (BSI) and other serious infections. In contrast, other Staphylococcus species, such as particularly Staphylococcus epidermidis, Staphylococcus haemolyticus, Staphylococcus hominis, Staphylococcus lugdenensis, and Staphylococcus saprophyticus commonly found on the skin, are with the exception of catheter-related infections rarely of clinical significance. Furthermore, in those instances where a BSI is caused by a Staphylococcus species other than S. aureusthere is often a difference in antibiotic susceptibility or viability between those organisms and S. aureus. Differentiation between Staphylococcus species other than S. aureus and S. aureus is therefore extremely important for directing appropriate patient therapy and patient management.
The presence of Staphylococcus species in clinical specimens is routinely determined by the presence of Gram-positive cocci in clusters (GPCC) by Gram staining and microscopic analysis, however; differentiation between S. aureus and other Staphylococcus species must await subculture, overnight incubation followed by biochemical analysis, such as coagulase and latex agglutination testing, or more recently by molecular testing. The presence of other Staphylococcus species is often determined on the basis of a negative tube coagulase test result and is typically reported as coagulase-negative staphylococci (CNS). This is particularly problematic for analysis of GPCC positive blood culture bottles as only 20-30% of GPCC positive blood bottles are due to S. aureus (Karlowsky et al., Ann Clin Microbiol Antimicrob 3:7 (2004)). In the majority of cases physicians therefore have to make decisions based on negative test results, e.g., absence of S. aureus. 
DNA probes for analysis of Staphylococcus aureus and all Staphylococcus species (genus-specific probes) have been described (WO0066788, Kempf et al., J. Clin. Microbiol 38:830-838 (2000)) as well as PNA probes for the analysis of S. aureus (U.S. Pat. No. 6,664,045, Oliveira et al., J. Clin. Microbiol. 40:247-251 (2002)). These probes all target sequences that are either species-specific or genus-specific.
Comparative analysis of ribosomal RNA (rRNA) sequences or genomic DNA sequences corresponding to said rRNA (rDNA) has become a widely accepted method for establishing phylogenetic relationships between bacterial species (Woese, Microbiol. Rev. 51:221-271 (1987)). Consequently, Bergey's Manual of systematic bacteriology has been revised based on rRNA or rDNA sequence comparisons. Ribosomal RNA or rDNA sequence differences between closely related species enable design of species-specific probes for microbial identification thus enabling diagnostic microbiology to be based on a single genetic marker rather than a series of phenotypic markers as in traditional microbiology (Delong et al., Science 342:1360-1363 (1989)). However, the design of probes targeting a cohort of species is particularly problematic and requires a combination of highly specific probe constructs and unique target sequences.
PNAs are useful candidates for investigation when developing probes targeting a subset of species because they hybridize to nucleic acids with increased sequence specificity as compared to DNA probes. Prior art therefore also comprises examples of PNA probes targeting mycobacteria other than Mycobacterium tuberculosis (U.S. Pat. No. 6,753,421) and enterococci other than Enterococcus faecalis (Oliveira et al., Abstract #D-2003, lnterscience Conference on Antimicrobial Agents and Chemotherapy, Sept. 27-30, 2002, San Diego, Calif.).
Despite its name, Peptide Nucleic Acid (PNA) is neither a peptide nor a nucleic acid, it is not even an acid. PNA is a non-naturally occurring polyamide that can hybridize to nucleic acids (DNA and RNA) with sequence specificity according to Watson-Crick base paring rules (See: U.S. Pat. No. 5,539,082) and Egholm et al., Nature 365:566-568 (1993)). However, whereas nucleic acids are biological materials that play a central role in the life of living species as agents of genetic transmission and expression, PNA is a recently developed totally artificial molecule, conceived in the minds of chemists and made using synthetic organic chemistry. PNA also differs structurally from nucleic acid. Although both can employ common nucleobases (A, C, G, T, and U), the backbones of these molecules are structurally diverse. The backbones of RNA and DNA are composed of repeating phosphodiester ribose and 2-deoxyribose units. In contrast, the backbones of the most common PNAs are composed on (aminoethyl)-glycine subunits. Additionally, in PNA the nucleobases are connected to the backbone by an additional methylene carbonyl moiety. PNA is therefore not an acid and therefore contains no charged acidic groups such as those present in DNA and RNA. The non-charged backbone allows PNA probes to hybridize under conditions that are destabilizing to DNA and RNA. Such attributes enable PNA probes to access targets, such as highly structured rRNA and double stranded DNA, known to be inaccessible to DNA probes (See: Stephano & Hyidig-Nielsen, IBC Library Series Publication #948. International Business Communication, Southborough, Mass., pp. 19-37 (1997)). PNA probes are not the equivalent of nucleic acid probes in structure or function.
Positive identification of Staphylococcus species other than S. aureus would be advantageous in many cases and simultaneous analysis of both S. aureus and other Staphylococcus species would be ideal as treatment decisions for the presence of either S. aureus or other Staphylococcus species would always be based on a positive test results. This feature would also offer a significant advantage when a mixture of S. aureus and other Staphylococcus species are present.